The kit of parts does come with an accelerometer and a gyro, plus "microswitches," basically touch sensors. But if you want to buy more, I know of
RobotShop,
Trossen Robotics,
Paralax, and yes, Spark Fun.
For gyro, I would go the full 360 since your robot will be going all over the place. As for an accelerometer, I wouldn't know; check the specs of FIRST's accelerometer for an example. It seems to be a 3g, 3 axis. Depending on what you are using the accel. for, you might get a 1 or 2 axis. But I would still recommend getting a 3 axis because you might need that capability in later years when you re-use the sensor.
Here's the
FIRST Sensor Manual, very helpful.
As for hooking up, I'm assuming either all/most sensors will be universal and the same as FIRST's sensors, but more likely they are only similar and there might be some differences. You'll have to look at the sensors you buy, see how they work and how you interface with them. If they won't work with the WPI libraries, there's always the digital input libraries that you can use to custom-receive information from the sensor. I don't think it will ever be that bad, though.
Hope it works out for you!